This invention relates in general to transresistance amplifiers (also referred to as xe2x80x9cI/V amplifiersxe2x80x9d or xe2x80x9ccurrent-to-voltage amplifiersxe2x80x9d) and, more specifically, to high bandwidth transresistance amplifiers.
As shown in FIG. 1, a conventional transresistance amplifier 10 used, for example, in a CD or DVD-ROM application includes a photo-diode 12 that receives laser light reflected off a CD or DVD-ROM disc. In response to the laser light, the photo-diode 12 generates an input current Ii. An operational amplifier 14 and a feedback resistor RF then convert the input current Ii into a voltage signal represented by an output voltage Vo. Specifically,
V0≈Iixc2x7RF+VREFxe2x80x83xe2x80x83(1)
where VREF is a reference voltage (e.g., ground) at the non-inverting input of the operation amplifier 14, and the gain of the amplifier 14 is assumed to be infinite.
Unfortunately, the photo-diode 12 typically has a relatively large parasitic capacitance Cp (about 2.5 pF) associated with it. As a result, the operational amplifier 14 has a dominant parasitic-capacitance-induced pole at its inverting input, which limits the bandwidth of the transresistance amplifier 10 to about 100 MHz. This limited bandwidth, in turn, tends to distort the voltage signal output from the amplifier 10, which makes the voltage signal harder to read and increases the possibility it will be read incorrectly.
Accordingly, there is a need in the art for an increased bandwidth transresistance amplifier.
In accordance with this invention, a circuit (e.g., a transresistance amplifier) for generating an output voltage from an input current includes a current amplifier that generates an amplified current in response to a bias current drawn from the current amplifier. Also, a current buffer includes an input node in communication with the input current, a current source that draws current from the input node, a buffering amplifier that has an input connected to the input node and an output that outputs a buffered voltage, and a buffering device for regulating the flow of bias current from the current amplifier to the input node in response to the buffered voltage. Further, an I/V amplifier coupled to the current amplifier receives the amplified current and generates the output voltage in response.
In another embodiment of this invention, an output voltage is generated from an input current when the input current is buffered by varying the output of a differential amplifier in accordance with the input current, and then varying the current flowing through a buffering device in accordance with the varying output of the differential amplifier. Also, the current flowing through the buffering device is amplified, and the output voltage is generated from the amplified current using a transresistance amplifier.